linux-kernel - RE: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi

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Message-ID:
 <LV3PR12MB9265DFD04F0F17DE7AAF204994F5A@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Mon, 20 Oct 2025 14:49:56 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Josh
 Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta
	<pawan.kumar.gupta@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, Dave
 Hansen <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, "H .
 Peter Anvin" <hpa@...or.com>, Alexander Graf <graf@...zon.com>, Boris
 Ostrovsky <boris.ostrovsky@...cle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Peter Zijlstra <peterz@...radead.org>
> Sent: Thursday, October 16, 2025 9:48 AM
> To: Kaplan, David <David.Kaplan@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Josh
> Poimboeuf <jpoimboe@...nel.org>; Pawan Gupta
> <pawan.kumar.gupta@...ux.intel.com>; Ingo Molnar <mingo@...hat.com>; Dave
> Hansen <dave.hansen@...ux.intel.com>; x86@...nel.org; H . Peter Anvin
> <hpa@...or.com>; Alexander Graf <graf@...zon.com>; Boris Ostrovsky
> <boris.ostrovsky@...cle.com>; linux-kernel@...r.kernel.org
> Subject: Re: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Thu, Oct 16, 2025 at 02:40:51PM +0000, Kaplan, David wrote:
>
> > > Can we please keep this in sync_core()? Something like:
> > >
> > > static __always_inline void sync_core(void)
> > > {
> > >         if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
> > >                 serialize();
> > >                 return;
> > >         }
> > >
> > > +       if (repatch_in_progress) {
> > > +               sync_core_nmi_safe();
> > > +               return;
> > > +       }
> > > +
> > >         iret_to_self();
> > > }
> > >
> > > That way all the modern stuff that has SERIALIZE will still use that.
> >
> > Hmm, I can't quite do that because sync_core() is used in a number of
> > other places too (unless we make repatch_in_progress a true global).
>
> We could just out-of-line the thing; nothing using this should care
> about cycles -- all of this is quite expensive.
>

Coming back to this, are you thinking we should just create something like 'text_poke_sync_core()' inside alternative.c and that can use:
   1. SERIALIZE (if available)
   2. MOV-CR2 (if re-patching)
   3. Else, IRET

And maybe someday we put MFENCE into there too for AMD parts.

Right now, of course this is the only logic that would care about an NMI-safe sync_core().  So maybe this makes sense vs creating a generic version that nobody else is using?

--David Kaplan